Various thermal transfer recording methods have hitherto been known in the art. Among others, a thermal dye transfer recording method, wherein a thermal transfer sheet comprising a sublimable dye-containing thermal transfer layer provided on a support, such as a polyester film, is heated with a heating medium, such as a thermal head or a laser, to form an image on a thermal transfer image receiving sheet, has recently attracted attention and has been utilized as information recording means in various fields.
The thermal dye transfer recording method can form a full-color image, in a very short time, that has excellent halftone reproduction and gradation and high quality comparable to that of full-color photographic images.
In order to receive the sublimable dye being transferred from the thermal transfer sheet and to hold the formed image, a receptive layer formed of a thermoplastic resin, for example, a saturated polyester resin, a vinyl chloride/vinyl acetate copolymer, or a polycarbonate resin, and, if necessary, an intermediate layer are provided on an image receiving surface.
For example, when a highly rigid substrate sheet, such as PET, is used, a layer for imparting cushioning properties, or a layer for imparting antistatic properties, is provided as the intermediate layer.
A backside layer formed by coating a composition comprising a binder, such as an acrylic resin, and, added thereto, an organic filler of an acrylic resin, a fluororesin, a polyamide resin or the like and an inorganic filler, such as silica, is optionally provided on the backside from the viewpoint of preventing curling and improving slipperiness of the thermal transfer image receiving sheet.
The so-called "standard type" thermal transfer image receiving sheet is used in such a manner that the image receiving sheet is viewed through reflected light rather than transmitted light. Also in this case, an opaque, for example, white, PET, foamed PET, other plastic sheet, natural paper, synthetic paper, a laminate of these materials or the like is used as the substrate sheet.
On the other hand, the so-called "seal type" thermal transfer image receiving sheet comprising a substrate sheet, a receptive layer provided on one side of the substrate sheet, and an adhesive layer, formed of a pressure-sensitive adhesive, and release paper provided on the other side of the substrate sheet has also been used in various applications. The seal type thermal transfer image receiving sheet is used in such a manner that an image is formed on a receptive layer by thermal transfer, the release paper is separated and removed, and the receptive layer with an image formed thereon is then applied to a desired object.
It is known that an antistatic layer of a surfactant or the like is formed on the surface of a thermal transfer image receiving sheet. In this case, however, problems occur such as creation of tackiness of the thermal transfer image receiving sheet, migration of the antistatic agent from the top surface to the back surface, and transfer of the antistatic agent onto a carrier roll or the like of a thermal transfer printer.
Further, these problems in turn create a problem of a lowering in antistatic effect with the elapse of time.
Another method is to form a conductive layer using a conductive agent of a metal oxide, such as conductive carbon black or tin oxide, and a binder. In order to obtain electrical conductivity, these conductive agents should be added in a considerably large amount. In addition, in many cases, these conductive agents inherently have black or other color. Therefore, basically, use of the above conductive agents in an image receiving sheet results in lowered whiteness of the image receiving sheet, making it impossible to use these conductive agents.
In order to solve the above problems, the formation of an antistatic layer using an acrylic resin having a quaternary ammonium base has been proposed. Japanese Patent Laid-Open No. 139816/1990 proposes a method wherein an antistatic layer is formed using these materials between a receptive layer and a substrate. Since, however, these materials have poor water resistance, use thereof in above manner results in remarkably lowered coating strength under high humidity (particularly high temperature) environment, leading to problems including that the coating is broken due to friction between the thermal transfer image receiving sheet and the roll during carrying at the time of printing.
Further, basically, these materials have poor adhesion to the substrate and other resins. Therefore, materials usable in this case are considerably limited. An additional problem is that the antistatic properties vary depending upon environment.